Thymerais (or Thimerais, French pronunciation: [timʁɛ]) is a natural region of Eure-et-Loir, in France, where history and geography meet.

Thymerais is also associated with the barony of Châteauneuf-en-Thymerais whose territory included in the thirteenth century the north-west of the Eure-et-Loir and some villages in Drouais, Eure and Orne, and overflowed the present townships of Courville-sur-Eure and La Loupe.

Open to influences from the Île-de-France, the Pays chartrain (Chartres region), Normandy and Perche, Thymerais consists of plateaus and valleys cleared in the Middle Ages, forests and ponds.

It was called "Land dismembered" because, after being joined to the Kingdom of France, it was separated from its royal domain and not, as is often believed, from the province of Perche.

The origin of Thymerais goes back to the 7th century, when king Thierry III gave this territory to Theodemer, prince of the Merovingian family.

The country was then called Theodemerensis (literally "Theodemer Territory") in his honor, then abbreviated to Themerensis and gallicized to Thymerais or Thimerais.

According to the vicomte de Romanet,[5][6] the Thymerais territory that was originally part of the counties of Chartres and Dreux, was occupied by powerful independent lords who owed allegiance only to the King of France.

In 1058, Albert Ribaud, lord of Thymerais, took a stand against William the Conqueror, Duke of Normandy and king of England, who seized Thimert and then installed there a governor.